This invention relates to array antennas, and more particularly to array antenna structures to aid in calibration of the active elements of the array.
Our society has become dependent upon electromagnetic communications and sensing. The communications are exemplified by radio, television and personal communication devices such as cellphones, and the sensing by radar and lidar. When communications were in their infancy, it was sufficient to broadcast radio signals substantially omnidirectionally in the horizontal plane, and for that purpose a vertical radiator or tower was satisfactory. Early sensors attempted to produce directional results, as for example the directional null used for direction-finding in the Adcock type of antenna. When it became possible to produce short-wave signals such as microwave signals efficiently and relatively inexpensively, directional results became possible with shaped reflector antennas, which provided the relatively large radiating aperture required for high gain and directionality. Such antennas have been in use for over half a century, and they continue to find use because they are relatively simple to build and maintain. However, the shaped-reflector antenna has the salient disadvantage that it must be physically moved in order to move the antenna radiated beam or beams.
Those skilled in the art know that antennas are reciprocal elements, which transduce electrical or electromagnetic signals between unguided (radiating-mode) and guided modes. The xe2x80x9cunguidedxe2x80x9d mode of propagation is that which occurs when the electromagnetic radiation propagates in xe2x80x9cfree spacexe2x80x9d without constraints, and the term xe2x80x9cfree spacexe2x80x9d also includes those conditions in which stray or unwanted environmental structures disturb or perturb the propagation. The xe2x80x9cguidedxe2x80x9d mode includes those modes in which the propagation is constrained by transmission-line structures, or structures having an effect like those of a transmission line. The guided-wave mode of propagation occurs in rigid waveguides, and in coaxial cable and other transmission-line structures such as microstrip and stripline. The guided-wave mode also includes transmission guided by dielectric structures and single-wire transmission lines. Since the antenna is a transducer, there is no essential difference between transmission and receiving modes of operation. For historical reasons, certain words are used in the antenna fields in ways which do not reflect contemporaneous understanding of antennas. For example, the term used to describe the directional radiation pattern of an antenna is xe2x80x9cbeam,xe2x80x9d which is somewhat meaningful in the context of a transmitting antenna, but which also applies to a receiving antenna, notwithstanding that conceptually there is no corresponding radiation associated with an antenna operated in its receiving mode. Those skilled in the art understand that an antenna xe2x80x9cbeamxe2x80x9d shape is identical in both the transmission and reception modes of operation, with the meaning in the receiving mode being simply the transduction characteristic of the antenna as a function of solid angle. Other characteristics of antennas, such as impedance and mutual coupling, are similarly identical as between transmitting and receiving antennas. Another term associated with antennas which has a contemporaneous meaning different from the apparent meaning is the definition of the guided-wave port, which is often referred to as a xe2x80x9cfeedxe2x80x9d port regardless of whether a transmitting or receiving antenna is referred to.
Array antennas are antennas in which a large radiating aperture is achieved by the use of a plurality of elemental antennas extending over the aperture, with each of the elemental antennas or antenna elements having its elemental port coupled through a xe2x80x9cbeamformerxe2x80x9d to a common port, which can be considered to be the feed port of the array antenna. The beamformer may be as simple as a structure which, in the reception mode, sums together the signals received by each antenna element without introducing any relative phase shift of its own, or which in the transmission mode of operation receives at its common port the signal to be transmitted, and divides it equally among the antenna elements. Those skilled in the art know that the advantages of an array antenna are better realized when the signal transduced by each elemental antenna of an array antenna can be individually controlled in phase. When phase is controlled, it is possible to xe2x80x9csteerxe2x80x9d the beam of the array antenna over a limited range without physical slewing of the structure. Introduction of phase shifters into the feed path of the elemental antennas, and for that matter the beamformer itself, necessarily introduces unwanted resistive or heating losses or xe2x80x9cattenuationxe2x80x9d into the signal path. These losses effectively reduce the signal available at a receiver coupled to the array antenna feed port in the reception mode of operation, and also reduce the power reaching the antenna elements from the feed port when in a transmission mode of operation.
In order to maximize the utility of array antennas, it is common to introduce electronic amplifiers into the array antenna system, to aid in overcoming the losses attributable to the beamformer and to the phase shifters, if any, and any associated hardware such as filters and the like. In an array antenna, one such amplifier is used in conjunction with each antenna element. For reception of weak signals, it is common to use an amplifier which is optimized for xe2x80x9clow-noisexe2x80x9d operation, so as to amplify the signal received by each antenna element without contributing excessively to the noise inherent in the signal received by the antenna element itself. For transmission of signals, a xe2x80x9cpowerxe2x80x9d amplifier is ordinarily associated with each antenna element or group of antenna elements, to boost the power of the transmitted signal at a location near the antenna elements. In array antennas used for both transmission and reception, both receive and transmit amplifiers may be used.
Amplifiers tend to be nonlinear, in that the output signal amplitude of an amplifier is in a specific amplitude ratio to the input signal amplitude at input signal levels lying below a given level, but become nonlinear, in that the ratio becomes smaller (the gain decreases to a value below the small-signal level) with increasing signal level. Structures which are subject to such saturation or other nonlinear effects are termed xe2x80x9cactive.xe2x80x9d It should be noted that an active element is often defined as one which requires or uses an electrical bias for operation; saturation tends to be inherent in such elements when the signal being handled approaches or equals the amplitude of the applied bias. Amplifiers are ordinarily not bidirectional, in that they amplify signals received at an input port, and the amplified signals are generated at an output port. Although bidirectional amplifiers are possible, the constraints required for bidirectional operation limit their utility, and unidirectional amplifiers are commonly used for array antennas. In the case of an array antenna used for both transmission and reception, each antenna element is associated with both a power amplifier and low-noise amplifier. Bidirectional, duplex or diplex operation, which is to say simultaneous operation in both transmission and reception, is accomplished by the use of circulators, which are three-port devices which allow connection of an antenna element to the output port of a power amplifier and to the input port of a low-noise amplifier. It should be noted that phase shifters which may be associated with each radiating element of an array in order to allow steering of the beam may be subject to saturation or nonlinear effects, and so may be considered to be xe2x80x9cactivexe2x80x9d for this purpose, although these nonlinear effects may not be nearly so pronounced as in the case of amplifiers, and in some cases the saturation effects of phase shifters may be ignored. Some types of phase shifters rely on the interaction of discrete electronic elements, which are affected by temperature and aging. Other types of phase shifters are almost immune to saturation effects, namely those using electronic switches to switch lengths of transmission line into and out of circuit.
One of the problems associated with the use of array antennas having active elements is that of changes in the characteristics of the active elements as a function of environmental conditions and of time. For example, the gain of an amplifier may change as a function of time or temperature, and the gain change can affect the beam formed by the beamformer in both transmission and in reception modes of operation, depending upon its location in the array antenna. Similarly, the inherent phase shift of an amplifier may change as a function of time or temperature, which in turn affects the net phase shift of the signal relating to that particular antenna element with which it is associated, which in turn affects the beam shaping or forming. The effects of aging and temperature on active devices associated with the elemental antennas of an active array antenna result in a requirement for calibration of the various active elements.
A difficult aspect of the calibration of the active elements of an array antenna is the determination of exactly what the characteristics of the active element(s) are, since the active elements tend to be xe2x80x9cburiedxe2x80x9d in the antenna structure. If attempts are made to physically access the input and output ports of the active elements, connections to the active elements must be made and broken for each active element, and the making and breaking of connections may itself introduce errors and changes to the system operation. Also, physical access to the active devices tends to be inconvenient due to the usual locations of the devices near the elemental antennas. U.S. Pat. No. 5,459,474, issued Oct. 17, 1995 in the name of Mattioli et al. describes an array antenna in which each radiating element is associated with one transmit-receive module, and the transmit-receive modules are mounted in racks which can be pulled out to expose the modules. While effective, such rack mountings tend to be relatively bulky, heavy, and expensive. U.S. Pat. No. 5,572,219, issued Nov. 5, 1996 in the name of Silverstein et al. describes a method for calibrating phased-array antennas by the use of a remote site and the transmission of orthogonal codes. U.S. Pat. No. 6,084,545, issued Jul. 4, 2000 in the name of Lier et al. describes a method for calibration of a phased-array antenna which eliminates the need for a distant source, and substitutes a near-field probe. Cooperative distant sources tend to be difficult to obtain at the desired time and location, and the near-field probes necessarily lie before the radiating aperture and perturb the desired fields.
Improved methods for calibration of phased arrays are desired.
An aspect of the invention lies in a method for calibrating the active elements of an array antenna used for transducing electromagnetic signal between unguided radiation and a guided transmission path. The active array antenna includes a beamformer including at least one guided-wave common port and at least N output ports associated with the common port. The guided-wave common port may be considered to be the xe2x80x9cfeedxe2x80x9d port for one beam of the array antenna. The antenna also includes a beamformer control computer coupled to the beamformer, for transducing signals therewith, and for forming beams based upon at least one of beamformer amplitude and phase transfer functions, and preferably both. The array antenna also includes a plurality of N radiating elements arranged in an array. Each of the radiating elements is capable of transducing electromagnetic signals with its own elemental port. A plurality of 2P calibration ports is provided, where P may be less than N in a preferred embodiment. P directional couplers are provided. Each of the P directional couplers includes first, second, third, and fourth ports, for coupling signal from the first port to the second and third ports and not to the fourth port, and from the second port to the first and fourth ports, but not to the third port. Each of the P directional couplers has its first port coupled to one, and only one, of the calibration ports, its second port coupled to another one, and only that one, of the calibration ports, its third port connected to a xe2x80x9ckernelxe2x80x9d one, and only that kernel one, of the N radiating elements, and its fourth port coupled to one, and only one, of the N output ports of the beamformer. As a result of these connections of P directional couplers to 2P calibration ports and P output ports out of N available output ports of the beamformer, Nxe2x88x92P=R non-kernel ones of the radiating elements lack a guided path to a directional coupler, and R ports of the beamformer are not connected to one of the directional couplers. The array antenna further includes a guided-wave connection between each of the R ports of the beamformer which are not connected to one of the directional couplers and a corresponding one of the R non-kernel radiating elements, as a result of which all of the N elemental antennas are connected to an output port of the beamformer, either through a directional coupler or through another guided-wave connection. At least one of (a) an active amplifier and (b) a controllable phase shifter is associated with at least some of the paths defined between the guided-wave common port and the at least N output ports associated with the common port of the beamformer.
According to another aspect of the invention, a method for calibrating the array antenna includes the step of applying a directional coupler calibration signal to a first one of the calibration ports, for thereby transmitting signal to a first port of a first one of the directional couplers, and in response to the step of applying of a directional coupler calibration signal, receiving returned directional coupler calibration signal at a calibration port coupled to the second port of the first one of the directional couplers. The amplitude and the phase of the returned directional coupler calibration signal are compared with the corresponding amplitude and phase of the calibration signal to establish a calibration transfer value for the guided-wave connection between the first one of the directional couplers and its associated calibration ports. The calibration transfer value may be compared with a predetermined or previously stored value, to thereby establish a directional coupler calibration reference value for the first one of the directional couplers. The next step in the calibration is to (a) apply beamformer calibration signal to the common port of the beamformer and extract corresponding beamformer calibration signal from that calibration port coupled to the second port of the first one of the directional couplers, or (b) apply beamformer calibration signal to that one of the calibration ports coupled to the second port of the first one of the directional couplers, and extract corresponding beamformer calibration signal from the beamformer common port, to thereby determine at least one of the amplitude and phase transfer between the common port of the beamformer and the fourth port of the first one of the directional couplers. As set forth in the claims, the terminology xe2x80x9cone of A and Bxe2x80x9d is slightly different from xe2x80x9ceither A or Bxe2x80x9d but has the same meaning, as understood by persons skilled in the art. From the calibration transfer value and from at least one of the amplitude and phase transfer between the common port of the beamformer and the fourth port of the first one of the directional couplers, at least one of the amplitude and phase characteristics of that signal path extending from the common port of the beamformer to the fourth port of the first one of the directional couplers are determined. The beamsteering control computer is adjusted by updating the parameters by which the control takes place, which may mean updating the value of the one of the amplitude and phase characteristic (or both) of that signal path extending from the common port of the beamformer to the fourth port of the first one of the directional couplers.
In a specific embodiment of an array antenna according to an aspect of the invention, the transmission-line electrical lengths extending between the calibration ports and the first and second ports of any one of the directional couplers are made or set equal, whereby the calibration transfer value for each of the cables is equal to one-half the calibration transfer value of the guided-wave connection to the one of the directional couplers.
A specific mode of the method according to the invention includes the further step of de-energizing all active elements of the beamformer except for those active elements lying in that path through the beamformer extending from the common port of the beamformer to a particular non-kernel one of the radiating elements of the array. This specific mode also includes the step of one of (a) applying beamformer calibration signal to the common port of the beamformer and extracting corresponding beamformer calibration signal from that one of the calibration ports associated with the first port of the first one of the directional couplers and (b) applying beamformer calibration signal to that one of the calibration ports associated with the first port of the first one of the directional couplers and extracting corresponding beamformer calibration signal from the common port of the beamformer, to thereby produce a nonkernel calibration signal including a measure of the mutual coupling between that one of the kernel radiating elements associated with the first one of the directional couplers and the particular non-kernel one of the radiating elements of the array. Finally, this specific mode includes the step of adjusting the beamsteering control computer by updating the parameters by which the control takes place by a factor responsive to the nonkernel calibration signal.